While organic electroluminescent devices have been known for about two decades, their performance limitations have represented a barrier to many desirable applications. (For brevity, EL, the common acronym for electroluminescent, is sometimes substituted.)
Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene", RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic emitting material was formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terpheyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic emitting material was present as a single layer having thicknesses above 1 mm.
The most recent discoveries in the art of organic EL device construction have resulted from EL device constructions with the organic luminescent medium consisting of two extremely thin layers (&lt;1.0 mm in combined thickness) separating the anode and cathode, one specifically chosen to inject and transport holes and the other specifically chosen to inject and transport electrons and also acting as the organic luminescent zone of the device. The extremely thin organic luminescent medium offers reduced resistance, permitting higher current densities for a given level of electrical biasing. Since light emission is directly related to current density through the organic luminescent medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g. brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
For example, Tang U.S. Pat. No. 4,356,429 discloses an EL device formed of an organic luminescent medium consisting of a hole injecting and transporting layer containing a porphyrinic compound and an electron injecting and transporting layer also acting as the luminescent zone of the device. In Example 1, an EL device is disclosed formed of a conductive glass transparent anode, a 1000 Angstrom hole injecting and transporting layer of copper phthalocyanine, a 1000 Angstrom electron injecting and transporting layer of tetraphenylbutadiene in poly(styrene) also acting as the luminescent zone of the device, and a silver cathode. The EL device emitted blue light when biased at 20 volts at an average current density in the 30 to 40 mA/cm.sup.2 range. The brightness of the device was 5 cd/m.sup.2.
A further improvement in such organic EL devices is taught by Van Slyke et al U.S. Pat. No. 4,539,507. Van Slyke et al realized a dramatic improvement in light emission by substituting for the hole injecting and transporting porphyrinic compound of Tang an aromatic tertiary amine layer. Referring to Example 1, onto a transparent conductive glass anode were vacuum vapor deposited successive 750 Angstrom hole injecting and transporting, 1,1-bis (4-di p-tolylaminophenyl)cyclohexane and electron injecting and transporting 4,4'-bis(5,7-di-t-pentyl-2-benzoxazolyl)-stilbene layers, the latter also providing the luminescent zone of the device. Indium was employed as the cathode. The EL device emitting blue-green light (520 nm peak). The maximum brightness achieved 340 cd/m.sup.2 at a current density of about 140 mA/cm.sup.-2 when the applied voltage was 22 volts. The maximum power conversion efficiency was about 1.4.times.10.sup.-3 watt/watt, and the maximum EL quantum efficiency was about 1.2.times.10.sup.-2 photon/electron when driven at 20 volts.
The organic EL devices have been constructed of a variety of cathode materials. Early investigations employed alkali metals, since these are the lowest work function metals. Other cathode materials taught by the art have been higher work function (4 eV or greater) metals, including combinations of these metals, such as brass, conductive metal oxides (e.g. indium tin oxide), and single low work function (&lt;4 eV) metals. Gurnee et al and Gurnee, cited above, disclosed electrodes formed of chrome, bass, copper and conductive glass. Dresner U.S. Pat. No. 3,710,167 employed a tunnel injection cathode consisting of aluminum or degenerate N.sup.+ silicon with a layer of the corresponding aluminum or silicon oxide of less than 10 Angstroms in thickness. Tang, cited above, teaches useful cathodes to be formed from single metals with a low work function, such as indium, silver, tin, and aluminum while Van Slyke et al, cited above, discloses a variety of single metal cathodes, such as indium, silver, tin, lead, magnesium, manganese, and aluminum.
Tang et al, U.S. Pat. No. 4,885,211 discloses an EL device comprised of a cathode formed of a plurality of metals other than alkali metals, at least one of which has a work function of less than 4 eV.
Commonly assigned VanSlyke et al U.S. Pat. No. 4,720,432 described an electroluminescent device using an improved multi-layer organic medium. As set forth in this patent the electroluminescent or EL device can be driven by a direct voltage source or an alternating current (AC) voltage source or any intermittent power source. This EL device is basically a diode rectifier which permits electrical current to flow only in the forward bias voltage. This current excites the organic medium to produce electroluminescence. In reverse bias, the current is blocked from entering the diode and consequently no light emission is produced.
Further improvement in organic electroluminescent devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos. 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 4,950,950; 5,104,740; 5,227,252; 5,256,945; 5,069,975, and 5,122,711.
Notwithstanding these improvements, there are still problems with the thermal stability of the EL devices comprising thin layers of vapor-deposited organic films. Thermal instability means that the EL device experiences faster degradation with increasing temperature or fails to function at a certain temperature above the room ambient. The cause of this instability is believed to be the morphological change in the organic layers used in the EL device. Furthermore, the change may initiate from any one of the organic layers, which is likely to be the one with the least thermal stability, to result in a complete device failure. It is clear from the prior art in organic EL that the hole-transporting material based on low-molecular-weight aromatic amines is the least thermally stable, characterized by a glass transition temperature generally below 100.degree. C. Therefore, it is important to further improve the thermal stability of this class of materials with a new design in the molecular structure. The expected advantages is that the EL device can be operated at a higher temperature. With a higher thermal degradation threshold, the EL device can also be driven to a higher brightness level because it is able to sustain a greater current density.